Recently, with development of material having effective corrosion resistivity under high temperature and pressure conditions, high temperature pressurized acid leach (high pressure acid leach) which uses sulfuric acid has been attracting attention as a hydrometallurgical process of nickel oxide ore (for example, see Patent Literature 1). This method does not include dry steps such as deoxidizing and drying step, but includes consistent wet steps unlike a conventional typical dry smelting method for nickel oxide ore and accordingly, the high pressure acid leach is advantageous in view of energy-saving and cost-performance. More specifically, in the high pressure acid leach, iron corresponding to a main impurity is fixed as leach residue in the form of hematite (Fe2O3) by controlling the oxidization-reduction potential and temperature of the leachate within the pressurized leach reaction vessel in the leach step. In this case, selective separation of nickel and cobalt from iron is allowed and this method is therefore considered extremely advantageous.
For example, high pressure acid leach using an autoclave is adopted as a hydrometallurgical process of nickel oxide ore. According to this method, during a high pressure acid leach step which includes leaching material slurry under high temperature and pressure conditions by using an autoclave and then reducing the temperature and pressure of the slurry after leach by using a flash vessel, the liquid level within the flash vessel is generally measured by a sensor directly attached to the flash vessel.
As can be seen from the general structure of a typical flash vessel 100 in FIG. 3, the flash vessel 100 includes a bottomed cylindrical body 101. A slurry inlet port 103 and a vapor outlet port 105 are provided at a ceiling portion 102 which closes the upper part of the body 101. In addition, a slurry outlet port 104 is provided at the body 101. A slurry inlet pipe 113, through which slurry after leach and reduction to predetermined temperature and predetermined pressure (hereinafter simply abbreviated as slurry in some cases) is introduced into the interior of the flash vessel 100, is coupled with the slurry inlet port 103. A slurry outlet pipe 114, through which the slurry having entered the interior of the flash vessel 100 is discharged, is coupled with the slurry outlet port 104. A vapor outlet pipe 115, through which vapor generated within the flash vessel 100 by introduction of the slurry is recovered, is coupled with the vapor outlet port 105. A slurry outlet valve 116 is provided on the slurry outlet pipe 114 coupled with the slurry outlet port 104.
According to the flash vessel 100, slurry after leach and reduction to predetermined temperature and predetermined pressure (hereinafter abbreviated as slurry in some cases) is introduced through the slurry inlet port 103, the slurry having entered the interior of the flash vessel 100 is discharged through the slurry outlet port 104 and the vapor generated by introduction of the slurry is discharged through the vapor outlet port 105.
In this case, the liquid level within the flash vessel 100 is maintained at an appropriate level based on measurement results of the liquid level within the flash vessel obtained by using liquid level sensors 120A and 120B.
For example, in the case of measurement of the liquid level using the liquid level sensor 120A for the maximum liquid level and the liquid level sensor 120B for the minimum liquid level, the slurry remaining within the flash vessel 100 is discharged by opening the slurry outlet valve 116 when the liquid level sensor 120A disposed at the maximum liquid level detects a rising liquid level. On the other hand, when the liquid level sensor 120B disposed at the minimum liquid level comes into a condition unable to detect the liquid level by a drop of the liquid level, the discharge of the slurry from the flash vessel 100 is stopped by closing the slurry outlet valve 116. Consequently, the slurry liquid level within the flash vessel 100 is adjusted within the range between the maximum liquid level and the minimum liquid level. For successive measurement of the liquid level, the discharge amount of the slurry remaining within the flash vessel 100 is raised by increasing the opening of the slurry outlet valve 116 when the liquid level exceeds the control liquid level, and reduced by decreasing the opening of the slurry outlet valve 116 when the liquid level becomes lower than the control liquid level.
In general, the leach reaction in the high pressure acid leach step is controlled based on control factors (pH and oxidation-reduction potential) of leach reaction produced by using a leaching agent, as well as based on temperature. For example, in a leaching method using chlorine gas as a leaching agent, the leaching reaction is controlled by oxidation-reduction potential in the leachate. In this case, the pressure within the autoclave is not directly controlled, and thus is not necessarily stable or constant during the leaching operation. Accordingly, the pressure is variable in accordance with the injective amount of chlorine gas controlled by the oxidation-reduction potential.
When the leaching agent is liquid and does not generate gas by reaction, the pressure within the autoclave is generally produced by saturated vapor pressure dependent on temperature. For example, in recent years, high pressure acid leach using an autoclave is adopted as a hydrometallurgical process of nickel oxide ore so as to recover valuable metal such as nickel and cobalt.
According to this high pressure acid leach, for example, ore slurry containing ore having a predetermined slurry concentration and a size of 2 mm or smaller is initially prepared by using pulverizing equipment and screening equipment and the ore slurry is supplied to the high pressure acid leach step. In this step, the temperature and pressure of the ore slurry are increased step by step by using a preheater (temperature and pressure raising equipment), and the resultant slurry is supplied to an autoclave. In this autoclave, nickel and cobalt contained in the ore, and a part of impurity elements such as iron, aluminum, and zinc are leached by using sulfuric acid to obtain slurry containing these materials after leach. Then, the slurry after leach is supplied from the autoclave to a flash vessel which reduces the temperature and pressure of the slurry after leach to the ordinary temperature and pressure, where the temperature and pressure of the slurry are reduced step by step. Thereafter, the slurry undergoes a preliminary neutralization step for neutralizing free sulfuric acid within the leachate, a solid-liquid separation step implemented by thickeners with a multiple—stage types, and other steps to be separated into leach residues and leachate.
The adoption of the flash vessel in the high pressure acid leach step eliminates the gap between the operation condition of the autoclave used in the high pressure acid leach step and that in the subsequent step. More specifically, the leach condition for the autoclave is generally adjusted to a temperature approximately in the range from 200° C. to 300° C. for obtaining high leach rate of nickel and cobalt. On the other hand, in the subsequent preliminary neutralization step or solid-liquid separation step, the operation is generally performed under the atmospheric pressure for safety and economic reasons. Accordingly, the flash vessel reduces the temperature and pressure while recovering pressurized vapor step by step from the high-temperature and high-pressure slurry after leach.
In the high pressure acid leach step, extremely expensive pipes constituted by material and structure resistant to high temperature and high pressure are provided in the piping for supplying slurry after leach from the autoclave to the flash vessel, the piping for supplying the recovered vapor to the preheater of ore slurry, the piping for increasing the temperature and pressure of the ore slurry step by step, and other piping. Accordingly, for meeting the demand for improvement of the entire cost-performance including the material cost, each piece of equipment is arranged in an appropriate position with the respective piping shortened. In this arrangement, the slurry after leach is transferred from the autoclave to the first step flash vessel, and then transferred sequentially to the subsequent flash vessels. The transfer of slurry after leach between the respective flash vessels is conducted by a method using the height differences of the places where the respective flash vessels are installed, and the pressure differences between the respective stages, rather than by adopting a mechanical transfer method such as a pump. This type of transfer is adopted in consideration of the durability and cost of the transfer equipment which transfers slurry after leach containing sulfuric acid. For example, according to a practical plant where an autoclave containing a cylindrical vessel having a size approximately in the range from 4 m to 6 m in diameter and approximately in the range from 25 m to 30 m in length is horizontally installed, the first stage flash vessel is located at a position corresponding to the height approximately in the range from 25 m to 35 m above the autoclave.
The pressurized vapor recovered step by step from the high-temperature and high-pressure slurry after leach is supplied from the flash vessels in the respective stages to preheaters having approximately the same temperature and pressure and the piping in this process is also provided with extremely expensive pipes made of material and structure sufficiently resistant to the pressurized vapor having high temperature and high pressure similarly to above.
However, the problem of damage to the vapor outlet pipe, the slurry outlet pipe, the valve and the like is not completely solved. During one-year operation, approximately ten times of troubles are caused in total, such as damage to the vapor outlet pipe, and damage to slurry outlet valve and accordingly, a practical technology capable of further reducing the problem of these failures has been demanded.
The estimated factor causing these problems is the insufficient control of the liquid level. More specifically, the slurry liquid level does not become flat in the condition where steam is generated by introduction of the high-temperature and high pressure slurry after leach into the flash vessel and rather than that, it is assumed that the liquid level is heavily variable by the steam generated from the depth of the slurry, in which condition the control of the liquid level is insufficient.
More specifically, in the high pressure acid leach step using the autoclave adopted as the wet smelting method for nickel oxide ore, the flash vessel, which reduces the temperature and pressure of the slurry obtained by leaching material slurry under high temperature and pressure conditions using the autoclave, is a large-sized flash vessel handling strong acid slurry. Accordingly, an inspection hole is difficult to be formed for technical reasons and visual inspection is substantially impossible.
Even when the actual liquid level in the conventional flash vessel 100 is high, for example, there is a possibility that the liquid level sensor 120A disposed at the maximum liquid level does not detect this condition due to considerable rise and drop of the liquid level. In this case, the liquid level control by the slurry outlet valve 116 does not work, and therefore operation is continued with the liquid level within the flash vessel 100 kept high and as a result, acid slurry is carried toward the preheater with the recovered vapor, whereby corrosion of the recovery vapor outlet pipe 115 may develop by the supply of the acid slurry. Similarly, even when the actual liquid level is low, there is a possibility that the liquid level sensor 120B disposed at the minimum liquid level does not detect this condition. In this case, the liquid level control by the slurry outlet valve 116 does not work, and the actual liquid level becomes lower than the slurry outlet pipe 114 and the vapor within the flash vessel 100 is discharged to the flash vessel disposed in the stage subsequent to the slurry outlet pipe 114 together with the discharged slurry, whereby the slurry flow speed within the outlet pipe temporarily increases and as a result, the slurry outlet pipe 114 and the valve may be broken, or the amount of the introduced vapor from the flash tank in the subsequent stage into the recovery vapor pipe temporarily increases. In this condition, corrosion and abrasion of the recovery vapor pipe may develop with increase in the carried sulfuric acid and increase in the flow speed.
For example, Patent Literature 2 discloses a technology relating to organic sludge slurry condensing method which constantly locates the liquid level of condensed liquid to an upper position than an outlet port based on detection of the liquid level within a flash vessel. However, this method is difficult to be adopted as it is due to the extremely different conditions in the point that the method is targeted to organic sludge slurry, and that the vapor pressure is only 2.5 atm, for example.
In addition, for example, Patent Literature 3 discloses a technology which controls refrigerant charge into a refrigerant vapor compression system by using at least one sensor provided to detect the level of liquid refrigerant within a flash vessel included in the system. However, this technology uses a sensor of float type or ultrasonic type, for example, which is applicable only when the liquid level is flat. Accordingly, this technology is difficult to adopt to the foregoing problems.